Silver is a valuable metal prized for use in jewellery, fine objects and coins, but it also has many industrial applications, particularly in photography. Some 75 per cent of recycled silver--which meets about 25 per cent of the world's needs for the metal--comes from photographic waste. Three different technologies are most commonly used for silver recovery.
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Electrolysis is the most commonly used technology to recover silver. Electric current is passed between positive and negative electrodes suspended in a silver-rich solution. Silver is electroplated onto the negative electrode, the cathode, and later removed. One advantage of electrolysis is that the silver it collects on the cathode is 90 per cent or more pure. A drawback of the method is that it requires substantial capital outlay. Another disadvantage of electrolysis is that extending the time the electrodes are in the solution--in an attempt to maximise the silver recovered--produces an inferior substance on the plating.
Metallic replacement is a process that involves passing a silver-bearing solution through a container filled with metal, typically steel wool. Sometimes other materials are used, such as iron-impregnated resins or iron screens, and commercial operations typically use a "metallic recovery cartridge." When silver comes into contact with the metal, it settles at the bottom of the container or deposits on the metal inside. These silver-bearing deposits are refined further. Initial capital costs for metallic replacement are modest and the equipment is easy to install, but it may not always reduce silver to legally acceptable levels.
Silver can be precipitated out of solution to very low levels using chemicals such as sodium sulphide and potassium sulphide. While this method has the advantage of being able to recover nearly 100 per cent of the silver, the silver precipitate obtained is difficult to filter. Careful monitoring is needed to prevent the creation of poisonous hydrogen sulphide gas during the process.
The ion-exchange method of recovering silver relies on a resin that will exchange its ions for those of silver in a solution. It works best on solutions of silver-bearing wastewater, but done properly, it can reduce the presence of silver in the wastewater to as low as 0.5 parts per million.
Reverse osmosis employs pressure to press silver in solution through a membrane that filters out the metal.
Photographic wastewater can be evaporated to form a sludge that is collected and refined further to recover the silver.
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